The invention herein is directed to a method of fabricating thin-film devices from a shape memory alloy, and to space-filling devices, e.g., intravascular devices, made by these methods.
Medical implants are increasing in use in minimally invasive surgery because of the improved medical results attainable. In particular, intravascular stents are used to reinforce blood vessels to promote healing and to prevent stenosis (narrowing) of blood vessels following procedures such as angioplasty. Alloys of titanium nickel (TiNi or Nitinol shape memory alloy) are gaining popularity over more traditional metals such as stainless steel for use in medical implants because the properties of shape memory and superelasticity enable improvements in design and methods of deployment of these devices. Demonstrated biocompatibility and novel methods of fabrication have resulted in wide acceptance of orthodontic braces, catheter guidewires, surgical tools, and implantable coronary stents.
Fabrication of stents from drawn TiNi tubes is practical only for a limited range of sizes. In particular, it has not been feasible to make stents having the flexibility and size required for delivery intravascularly through small catheters via the carotid arteries.
There is a growing demand for smaller and thinner, more flexible stents that can be surgically implanted or delivered via catheter, into small diameter, highly tortuous blood vessels. Suitably flexible structures can be fabricated of thin film (2-10 micrometers thick) shape memory alloys that are sputter deposited on a substrate and heat treated. Composition and heat treatment affect the phase transition temperature of the alloy, which in turn determines whether it exhibits shape memory or superelastic properties.
For maximum effectiveness, an intracranial device should be installed through a small diameter catheter, then changed to a pre-determined shape so as to fill a space and apply continuous outward pressure against the blood vessel wall. To accomplish this, three-dimensional shapes such as cylinders, cones, and hemispheres are required, and a shape-changing capability is highly advantageous.
The invention includes, in one embodiment, a thin film device comprising a seamless thin-film expanse (i) formed of a sputtered Nitinol shape memory alloy; (ii) defining, in an austenitic state, an open, interior volume;(iii) having a thickness between 0.5-100, preferably 2-50 microns; (iv) having an austenite finish temperature Af below 37xc2x0 C.; and (v) demonstrating a stress/strain recovery greater than 3% at 37xc2x0 C. The expanse can be deformed into a substantially compacted configuration in a martensitic state, and assumes, in its austenitic state, a shape defining such open, interior volume. The expanse may have, for example, a cylindrical, hemispherical or sock-like shape.
The device may include a skeletal member to which the expanse is attached, and these members may have a thickness greater than the thickness of the expanse. In addition, the expanse may be fenestrated with a selected pattern of openings in the thin film.
In another aspect, the invention includes a method of forming the thin-film device. The method includes the steps of placing in a magnetron sputtering device, a mandrel having an exposed, etchable outer layer that corresponds to the open, interior volume of the device to be formed, providing the sputtering apparatus with a TiNi alloy target composed of between 45-55% each of titanium and nickel, and sputter depositing material from the target adjacent said mandrel under low-pressure, low-oxygen conditions. During the deposition, the mandrel is moved relative to said target, to achieve substantially uniform sputter deposition over the entire exposed surface of the mandrel, and the deposition is continued until a desired sputtered film thickness between 0.5 and 100 microns, preferably 2 and 50 microns, is formed on the mandrel.
Following sputter deposition, the thin film on the mandrel is heated under annealing conditions. The thin-film device so formed is then released from the mandrel, typically by exposing the mandrel and deposited thin film to an etchant, under conditions effective to dissolve the outer layer of the mandrel. The mandrel""s outer layer may be a separate coating formed on the mandrel surface, or the surface of the mandrel itself. The mandrel may be coated with a smooth surface such as polyimide before sputtering to ensure a continuous layer of deposited material.
The target has a preferred composition of between about 48 to 51 atomic percent nickel to 52 to 49 atomic percent titanium. Where the sacrificial layer material is chromium, aluminum, or copper, and the etchant may be a chrome etch, potassium hydroxide, and nitric acid.
The mandrel is preferably rotated during the sputtering step to achieve substantially uniform sputter deposition over the entire exposed surface of the mandrel.
In various embodiments the mandrel may be cylindrical, e.g., for producing a thin-film stent, sock-like, e.g., for producing an intravascular filter, or hemispherical, e.g., for producing a vaso-occlusive device.
The method may further include applying structural members to the mandrel, prior to depositing the thin film thereon, to form structural members in the formed device. For use in forming a fenestrated thin-film device, the method may further include forming on the annealed thin film, an resist layer containing a pattern of openings, exposing the coated thin film with a solvent under conditions effective to create fenestrations in the thin film corresponding to the pattern of openings, and removing the resist layer. The fenestrations may have dimensions and interfenestration spacings in the 10-50 micron range.
These and other objects and features of the invention will be more fully appreciated when the following detailed description n of the invention is read in conjunction with the accompanying drawings.